1. Field of the Invention
The present invention relates to an improvement of a vehicle steering apparatus utilizing steer-by-wire technology.
2. Related Art
As a typical steering apparatus for vehicles, conventionally, there has been known a steering apparatus which turns wheels through a turning mechanism coupled to a steering wheel by a steering effort of the steering wheel.
Further, there has been recently known a vehicle steering apparatus utilizing steer-by-wire (hereinafter referred to as “SBW”) that mechanically separates a turning mechanism from a steering wheel, makes a turning actuator (a turning-power motor) generate turning power in accordance with a steering amount and that transmits the turning power to the turning mechanism to wheels.
According to a steering device for vehicles proposed in Japanese Unexamined Patent Application Publication No. 2005-29016, controlling a turning-power motor by a controller permits automatic setting of the characteristics of the angle ratio of the turning angle of a wheel to the steering amount (the steering angle) of the steering wheel, that is, the steering characteristics (turning angle-steering angle ratio).
FIG. 4 is a view illustrating the relationship between vehicle speeds and turning angle-steering angle ratio in a conventional vehicle steering apparatus utilizing SBW. As illustrated in FIG. 4, the turning angle-steering angle ratio is the highest when the vehicle speed is zero and decreases as the vehicle speed increases. This increases the turning angle of a wheel relative to a steering angle of a steering wheel at low speeds, thus improving tight cornering, and decreases the turning angle of a wheel relative to the steering angle of the steering wheel at high speeds, thus improving vehicle stability.
However, in performing steering at a vehicle speed of zero, that is, stationary steering during stopping, a road wheel is turned without being rotated, accordingly, the maximum turning power is required for road wheel turn. In a conventional vehicle steering apparatus utilizing SBW providing the maximum turning angle-steering angle ratio at a vehicle speed of zero, the turning angle of the road wheel relative to the steering angle of the steering wheel becomes maximum at a vehicle speed of zero, therefore, the turning-power motor requires the highest speed to turn the road wheel. Accordingly, ensuring the turning power necessary for stationary steering unavoidably required an enlargement of the turning-power motor. Because the turning-power motor is expensive, enlargement of the turning-power motor may increase the manufacturing costs of the steering apparatus. Moreover, the turning-power motor must be installed in a small vehicular space, for which size reduction is in demand.
In view of the above-described problems, it is an object of the present invention to provide a technology capable of downsizing the turning-power motor a in a vehicle steering apparatus utilizing SBW.